DocBook provides a system for writing structured documents using SGML or XML. It is particularly well-suited to books and papers about computer hardware and software, though it is by no means limited to them. DocBook is an SGML document type definition (DTD). An XML version is available now, and an official XML release is in the works. Because it is a large and robust DTD, and because its main structures correspond to the general notion of what constitutes a book, DocBook has been adopted by a large and growing community of authors. DocBook is supported "out of the box" by a number of commercial tools, and support for it is rapidly growing in a number of free software environments. In short, DocBook is an easy-to-understand and widely used DTD. Dozens of organizations use DocBook for millions of pages of documentation, in various print and online formats, worldwide.